1. Field of the Invention
The invention relates to a method for the dimming of the light emitted from LED lights, and in particular, in the passenger cabin of an airliner.
2. Discussion of the Prior Art
DE 10 2005 016 729 B3 discloses the dimming of the light emitted from a white-light light-emitting diode (LED) in successive working periods without any gaps and of the same length as one another, in each of which high-frequency chopping takes place of the current which flows during the switched-on time intervals in the successive working periods through the diode. The shorter the switched-on time interval in the working period is, the fewer constant-current pulses flow through the LED and in consequence the lower is the brightness of the emitted light.
In order to vary the color impression of an LED light, the light emitted from LED arrays in the primary colors red, green and blue is normally superimposed with different intensity, for which purpose the individual arrays have their array-current time intervals controlled independently of one another in the working periods, for dimming purposes.
However, only a dimming ratio in the order of magnitude of 1:1000 between dark and bright can be achieved in this way. This is no longer sufficient for, for example, constant-color variable dimming impressions (for example the extended transition over time from starlit heavens to sunrise in the case of the lighting in a passenger cabin) with gamut color correction (compensation for the shift to a warmer light color during the transition to reduce brightness), when the RGB light-emitting diode arrays are already being operated in a highly dimmed form, that is to say at a very low brightness which can be adjusted in this way; the aim is to achieve a dimming ratio that is greater than this by at least one order of magnitude to allow operation at even lower levels, before being completely switched off.
This is because in gamut color correction, which is required for high-quality, constant-color lighting effects, is dependent on very short current-flow times through light-emitting diodes. This is because it is then possible to compensate for variation of the color loci of LEDs within a production batch. Specifically, in order nevertheless to achieve a specific primary color, with two other primary colors are mixed in with low intensities even during the production matching process or later during operation (controlled by photodiodes), as a result for the respective color locus written from the color triangle, as written in the CIE standard color table (into what is also referred to as the color shoe) for the LEDs. For example, a gamut-corrected guaranteed color locus of “blue, unsaturated” is produced by driving the green LED at 5% and the red LED at 2%, in addition to the blue LED being driven at full power (100%). In order to present this color locus with a low brightness, for example dimmed to 1%, with a drive cycle of 3 ms, this results in the blue being switched on for a time of 1% of the full cycle, that is to say 30 μs, the green being switched on for 1% of 5%, that is to say 0.05% (1.5 μs), and the red being switched on for 1% of 2%, that is to say 0.02% (0.6 μs current flow through the red LED).
Passing current pulses that are as short as this through LEDs results in numerous problems. For example, these short pulses have fundamental frequencies of several hundred kilohertz, and this can lead to disturbing interference (electromagnetic interference) and frequencies which are allocated to specific radio services (for example the emergency radio at 200 kHz); excessively short-switched-off times make it difficult to discharge the natural capacitances within the LEDs; and it is not possible to produce current sinks which switch sufficiently quickly using low-cost components. Such extreme LED dimming would be feasible from the circuitry point of view only by using very fast and therefore expensive processes with a high coding depth for the fine subdivision of the working period, together with high-power, radio-frequency transistors for the current sinks in the R, G and B diode series circuit, that is to say with a rarely acceptable level of circuitry complexity.